1. Field of the Invention
This invention relates to a superplastic Mg-based composite material reinforced with titanium carbide particles, aluminum nitride particles, silicon nitride whiskers or particles, and titanium boride particles and a method for the production thereof.
2. Description of the Prior Art
Magnesium alloys are the lightest and the highest in strength per unit volume among all practical metallic materials. Like aluminum, they have melting points around 650.degree. C. Magnesium-based composite materials reinforced with ceramic whiskers or particles excel aluminum composite materials in terms of specific strength and specific modulus of elasticity and are excellent in abrasion resistance, dimensional stability at elevated temperatures, and thermal conductivity. Moreover, magnesium alloys are capable of absorbing vibration. Thus, magnesium-based composite materials are expected to find utility in structures and mechanical components in the aerospace industry and the field of transportation equipment.
Particularly when utilized in the aerospace industry, these composite materials have to be press formed into three-dimensional structures, most of which have complicated shapes and large surface areas. It is, therefore, necessary that the composite material be capable of being rolled into a thin plate, the resultant thin plate be capable of being pressed into a desired shape, and the formed article consequently obtained exhibit better material quality than the material prior to undergoing the formation. For meeting this need, attempts are being made to endow magnesium-based composite materials with superplasticity.
The heretofore developed composite materials reinforced with superplastic ceramic whiskers or particles use a matrix of aluminum. Examples of well-known superplastic magnesium alloys include such Mg-Li type alloys as Mg-9% Li alloy and Mg-8.5% Li alloy (G. Gonzalez-Doncel, D. A. Ruans, J. Wolfenstine and O. D. Sherby: Materials Science and Engineering, 125A (1990) 195 p and K. Higashi and J. Wolfenstine, Materials Letters, IO-1/8 (1991) 134/137 p). These alloys are reported to have matrix crystal grain diameters in the range of from 6 to 35 .mu.m and exhibit a maximum total elongation in the range of from 460 to 600% at a strain rate in the approximate range of from 3 to 4.times.10.sup.-3 S.sup.-1 at a temperature in the range of from 453 to 623K. These superplastic properties are practically equal to those of the conventional superplastic aluminum alloys and fall short of satisfying the conditions for a high-speed superplastic material.
As magnesium composite materials reinforced with ceramic whiskers or particles, magnesium-based composite materials reinforced using SiC whiskers or particles are known to the art (Jun Sun Kim, Junichi Kaneko, and Makoto Sugamata: J. Japan Inst. Metals, 56-7 (1992) 819/827 p). Methods available for the production of these magnesium-based composite materials include the forging cast, the melt stirring and mixing (vortex) method, the compocasting method, the powder metallurgy method, and the foil metallurgy method.
A process has been studied for superplasticizing a magnesium-based composite material by using the forging cast to manufacture a magnesium composite material reinforced with SiC whiskers and extrusion forming the composite material in a prescribed shape (J. Japan Inst. Metals, 56-7 (1992) 819/827 p). The composite material obtained by the process, however, does not attain as a large elongation as expected, probably because of high reactivity between SiC and Mg. No magnesium-based composite material using a ceramic substance other than SiC as a reinforcing material has been reported to achieve superplasticity.
In the development of a method for the production of a Mg-based composite material, it is necessary to ascertain the optimum process in light of the properties of Mg.
The powder metallurgy method, for example, is not practicable because magnesium powder is explosive and highly dangerous to handle. The casting method has to be carried out in the ambience of an inert gas because a molten magnesium alloy is susceptible of oxidation. In the course of stirring the alloy melt in this method, it is not easy to thoroughly mix the reinforcing material with the melt in the form of a fine powder of a particle diameter of about 1 .mu.m. Further, since the melt being stirred extrains ambient gas, the produced composite material is apt to include numerous defects. Magnesium-based composite material produced by the casting method has therefore been regarded as inferior. The stirring and mixing method, however, proves most suitable for the production of commercial machine parts because it allows composite materials to be manufactured in large quantities at a low cost.
At present, superplastic Mg-containing composite materials having varying properties are in demand from the practical point of view. The present inventors pursued a study with the object of meeting this demand. This invention has been accomplished as a result.